We propose to investigate the role of circadian rhythms in photoperiodic time measurement. One of the goals is to establish the relation between biological clocks, photoperiodism and seasonal reproductive activity in representative mammals. The participation of the nervous and endocrine systems in these phenomena will be detailed. The overall goal is to describe the ways in which temporal integration of the animals' internal milieu is affected and the fashion in which behavior is synchronized with cyclical fluctuations in the external environment. BIBLIOGRAPHIC REFERENCES: Morin, L. P., Fitzgerald, K. M. and Zucker, I. Estradiol shortens the period of hamster circadian rhythms. Science, 1977, 196, 305-307. Bittman, E. L., and Zucker, I. Influences of the adrenal gland and photoperiod on the hamster oestrous cycle. J. Reprod. Fertil, 1977, in press.